Rat skeletal muscle glycogen degradation pathways reveal differential association of glycogen-related proteins with glycogen granules

Glycogenin, glycogen-debranching enzyme (GDE) and glycogen phosphorylase (GP) are important enzymes that contribute to glycogen particle metabolism. In Long-Evans Hooded rat whole muscle homogenates prepared from extensor digitorum longus (EDL, fast-twitch) and soleus (SOL, oxidative, predominantly slow twitch), it was necessary to include alpha-amylase, which releases glucosyl units from glycogen, to detect glycogenin but not GDE or GP. Up to similar to 12 % of intramuscular glycogen pool was broken down using either in vitro electrical stimulation or leaving muscle at room temperature > 3 h (delayed, post-mortem). Electrical stimulation did not reveal glycogenin unless alpha-amylase was added, although in post-mortem muscle similar to 50 and similar to 30 % of glycogenin in EDL and SOL muscles, respectively, was detected compared to the amount detected with alpha-amylase treatment. Single muscle fibres were dissected from fresh or post-mortem EDL muscles, mechanically skinned to remove surface membrane and the presence of glycogenin, GDE and GP as freely diffusible proteins (i.e. cytoplasmic localization) compared by Western blotting. Diffusibility of glycogenin (similar to 20 %) and GP (similar to 60 %) was not different between muscles, although GDE increased from similar to 15 % diffusible in fresh muscle to similar to 60 % in post-mortem muscle. Under physiologically relevant circumstances, in rat muscle and within detection limits: (1) The total cellular pool of glycogenin is always associated with glycogen granules, (2) GDE is associated with glycogen granules with over half the total pool associated with the outer tiers of glycogen, (3) GP is only ever weakly associated with glycogen granules and (4) addition of alpha-amylase is necessary in order to detect glycogenin, but not GDE or GP.